The use of waterborne polymer dispersions in the coatings industry has increased significantly due to an industry effort to develop coating formulations containing little or no volatile organic compounds (VOC). Moving from organic solvent-based compositions to aqueous compositions brings health and safety benefits. However, coating manufacturers and customers desire aqueous coating compositions that achieve the same as or better performance standards than previous solvent-based compositions.
Waterborne polymer dispersions have been prepared from various polymers including vinyl polymers, acrylic polymers, and alkyd polymers. In use, alkyd polymers tend to exhibit excellent film forming ability and adhesion due to their low molecular weight, flexibility, and oxidative self-crosslinking. As a result, a film coating comprising an alkyd resin has very high gloss. However, alkyd polymers have relatively slow cure times, particularly at ambient temperatures. In contrast, vinyl and acrylic polymers have fast cure times but suffer poor gloss and adhesion. For the vinyl and acrylic polymers, attempts to enhance gloss and adhesion by lowering the glass transition temperature of the vinyl and acrylic polymer film often leads to film softness and poor dirt pick-up properties. Therefore, coating manufacturers seek to impart alkyd attributes to vinyl and acrylic polymers.
For example, U.S. Patent Application Publication 20110144258 describes waterborne polymer combinations that physically blend acrylic polymers with waterborne alkyds. However, such combinations typically fail to provide the desired performance due to phase incompatibility between alkyd and acrylic polymers, thus leading to a large scale phase separation in the micron range during film formation. Avoiding a large scale phase separation during film formation is desirable. Further, U.S. Pat. No. 6,333,378 discloses using a mini-emulsion process to produce hybrid alkyd/acrylic emulsions. The mini-emulsion process requires the use of specialized equipment such as a microfluidizer.
In U.S. Pat. No. 6,333,378, the latex particles comprise a core/shell structure wherein the core comprises an acrylic-based polymer and an alkyd polymer and the shell comprises a surfactant. The alkyd polymer is in the core of the particle rather than the shell.
U.S. Patent Application Publication 20110144258 teaches the physical blending of an alkyd polymer with an acrylic-based polymer after the acrylic based polymer has already been formed (i.e., after polymerization).
The present inventors have found that using an alkyd polymer as a co-surfactant during polymerization of an acrylic or vinyl monomer results in improved film formation properties for the latex that is produced.
The inventors further surprisingly found that configuration of the latex particle, and in particular, the placement of the alkyd polymer in the shell of a core-shell particle favorably impacts performance properties of a latex formed with the latex particles.
In the latex particles of the present invention, the acrylic modified alkyd polymer is located in the shell of the core/shell polymer and coats the core of the particle.
In the latex particles of the prior art that are produced using processes wherein post-polymerization addition of an alkyd polymer occurs, the alkyd polymer does not coat the acrylic-based polymer, thereby resulting in a large scale, micron range phase separation, therefore less desirable film formation properties.